Audio compressor – How does it work

Waves SSL compressor plug-in

Waves SSL compressor plug-in

If you’ve been mixing for a while you’ve probably noticed the audio compressor in your plug-in library. Compressors are used in the recording studio, live on stage and in every song you hear on the radio. They are essential for a good sounding mix and a fine tuned audio master. In this article I will explain how the audio compressor works and how to use it in a proper way. This does not mean blindlydropping an audio plugin without knowing what you’re exactly doing, like a lot of amateurs are doing. Use them only when they are needed and adjust them the proper way.

How to set the audio compressor in a proper way

Compression of audio is one of the most important and most underrated techniques for mixing audio. It is used to reduce peaks in the audio signal to a certain level. Simply put, if the sound level reaches above a threshold the volume of the audio from that point will be reduced with a certain ratio. Let us listen to a singer as an example. The volume of his voice in the verse is fairly mild with little dynamics. In the chorus you’ll hear a difference in volume with a lot of dynamics. If a compressor is properly set, these loud parts in volume in the chorus are reduced. So you therefore have control over the dynamics of the singer’s voice. In other words, with compression you don’t have volume peaks in the mix. Very useful, isn’t it? Now we only need to know the correct settings.

The settings of the compressor

Basically you can adjust 5 important settings:

  1. Threshold
  2. Compressor Ratio
  3. Make up gain
  4. Attack
  5. Release

 

I will explain each function of the compressor

1. Threshold

This is the limit above which the compressor works. All audio above this level will be compressed according to the compressor settings.

2. Ratio

This is the amount of compression. Choosing a ratio of 2:1 will reduce an 8 dB peak above the threshold with 4 dB. Choosing a ratio of 4:1 will reduce this peak with 6 dB. A compressor with a ratio of 1:infinite is called a limiter.

Audio compressor ratio settings

Audio compressor ratio settings

3. Make up gain

Because the overall sound level is reduced in audio compression you can correct this with the make up gain. If you use a ratio of 3:1 with an average sound level above the threshold of 9 dB. Then you should set the make up gain to 6 dB (think about it how this works). Divide the 9 dB above threshold by 3 (the ratio) and the answer is 3, this is the peaklevel above the threshold after complression. So the output level is reduced by 6 dB (9 dB – 3 dB) and we have to correct it with the make-up gain by setting it to 6 dB (Did you get it? good!).

4. Attack

This is the speed at which the compressor should work. With a very fast attack, the audio above the threshold is very rapidly compressed. Thus, a percussive sound, such as a snare drum, will lose its punch. With a slow attack it will remain its punch. You should try this out.

5. Release

This is the opposite of the attack. Thus the speed at which the compressor stops. Experiment with the release and you will notice the difference in the character of the sound. i.e. If you want an instrument to have more sustain you should use a fast release time.

Some compressor settings

Bass instruments

  • Ratio of 4:1 to 8:1
  • Attack: slow
  • Release: slow

 

Give an instrument more sustain

  • Ratio of 8:1
  • Attack: fast
  • Release: fast

 

Drums and Percussion

  • Ratio of 12:1
  • Attack: fast
  • Release: Fast to medium

 

You could use this to maximize the volume before it is being recorded to tape. You will yield a higher gain level of the preamplifier. Use this setting only on percussive instruments like a kick, a snare or a piano.

Volume leveling

  • Ratio of 2:1 to 4:1
  • Attack: average
  • Release: average

 

This setting is used for audio with slow but constant volume fluctuations and would be appropriate for vocals. If the intensity of volume changes are louder, then you should use a higher ratio or lower the threshold until the fluctuations in the volume are leveled.

How to set a side chain on a compressor

The use of a side chain on a compressor is very popular nowadays. The so-called pumping effects in dance tracks are being created this way. Very simple. The compressor turns on when it is triggered by the kick drum and how to create this effect is fairly easy.

  1. Send the signal of the kick to i.e. audio bus 1
  2. Group some instruments while sending them to bus 2
  3. Install the compressor on bus 2
  4. Set bus 1 (the kick) as a side chain for the compressor
  5. Adjust the threshold, ratio, attack and release of the compressor to the desired sound effect

 

Now you know how to use an audio compressor for mastering and mixing purposes. I hope this article was very useful for you. All information on this website is free. In return you can help us by creating a back link to track-mastering.com.